Control (automatic)

A control in Automatique is an algorithm whose main object is to stabilize and to improve the reaction of a system compared to its instruction. The general principle is to compare the instruction and the state of the system so as to correct it. One also speaks about system ordered by negative Rétroaction, or about loop system closed.

Principle

The basic principle of a control is to measure the difference between the actual value of the physical size to control and the set point which one wishes to reach, and to calculate the suitable order applied to one (or of) Actionneur S in order to reduce this variation.

This practically universal principle has a disadvantage however (generally negligible and neglected, except for the system very fast or requiring precise follow-ups of trajectory): the actuators being controlled according to the difference between actual value and instruction, the system can react only when this variation is effective, i.e. with a certain delay. One can, in certain cases, to compensate for this effect by supplementing control by a feed forward , i.e. by controlling the actuators to obtain the target value directly, or at least by giving the order which makes it possible to approach some more as a blind man, if one can envisage the behavior of the actuators. For example one can, in the case of a position control, to directly control acceleration if the acceleration of the instruction is known.

Performances

Several parameters characterize the performances of a control:

the speed to which the end value will be reached. It is the response time .
the stability . A system is stable if the exit tends towards a finished value. If it oscillates, control is unstable.
the going beyond . Often expressed pourcent some. Even when a system is stable, it happens that the exit exceeds the instruction before being stabilized.
the precision , i.e. capacity of control to reach the instruction with precision.

A control acts on several characteristics of the size controlled like:

the position: Control of position
speed: Control speed
acceleration: Control of acceleration

Certain complex systems optimize several of these characteristics to obtain at the same time fast and precise answers. For nonmechanical systems, one can also control other types of size: tension, phase… The principal constraint being of being able to measure them and to act on them by an effective order.

See too

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